Method and apparatus for determining the position or dimensions of a test object

ABSTRACT

An improved apparatus and process for sensing a test object by means of a switching sensor which is movable relative to the test object, in which the relative position between the test object and the switching sensor is measured by a measuring instrument. The switching sensor generates a switching signal at the moment of contact with the test object in order to cause the instantaneously prevailing measuring value of the measuring instrument to be retained. Such measuring values commonly are characterized by sensing errors as a function of the sensing speed and/or the material pairing between the switching sensor and the test object. In order to correct for such sensing errors the measuring value present at the moment of contact between the switching sensor and the test object is added with previously determined correction values for correcting errors resulting from the velocity of approach between the test object and the switching sensor and/or for the material pairing between the switching sensor and the test object.

BACKGROUND OF THE INVENTION

The present invention relates to a method and apparatus for determiningthe position and dimensions of a test object by means of a measuringsystem coupled to a switching sensor, wherein the switching sensor ismovable relative to the test object with an approach velocity, whereinthe measuring instrument generates a measuring signal indicative of therelative position of the switching sensor with respect to the testobject, and wherein the switching sensor generates a sensing signal uponcontact with the test object.

European Patent Application No. 0 029 499 discloses an arrangement fordetermining the dimensions of a test object by means of a switchingsensor movable relative to the test object. A measuring arrangement isincluded for measuring the relative position between the test object andthe switching sensor. At the moment of contact between the switchingsensor and the test object, the switching sensor generates a sensorsignal which causes the instantaneously prevailing measuring value ofthe measuring instrument to be stored.

Typically, during operation the switching sensor approaches the testobject with an approach velocity that can vary widely. It has been shownthat in systems of the type described above the measured position of thetest object is affected by errors caused by deformation of the switchingsensor and of the test object at the common contact point between them.Such deformations vary in the first place as a function of the sensingspeed or velocity of approach between the switching sensor and the testobject and, in the second place, as a function of the materials used forthe switching sensor and test object.

SUMMARY OF THE INVENTION

The present invention is directed to an improved apparatus and methodfor determining the position and dimensions of a test object byeliminating measuring errors of the type described above dependent uponsensing speed and material paring.

According to this invention, a measuring system of the type describedabove is modified by first storing a set of correction values indicativeof previously determined switching sensor errors resulting from at leastone of (1) the velocity of approach between the test object and theswitching sensor and (2) material characteristics of the switchingsensor and the test object. The sensing signal is used to store a valueof the measuring signal, and this value of the measuring signal is thencorrected with at least a selected one of the correction values in orderto reduce errors.

The present invention provides the important advantage that previouslydetermined correction values can be used to provide the desiredcorrections in a remarkably simple, rapid, and accurate manner. In thisway, the measured dimension of the test object can be made substantiallyindependent of the particular sensing speed and the particular materialpairing between the switching sensor and the test object. Thus, theposition and dimensions of the test object can be measured in a highlyefficient manner while avoiding errors of the type described above.

Further advantageous features of the invention are set forth in thedependent claims.

The invention itself, together with further objects and attendantadvantages, will best be understood by reference to the followingdetailed description, taken in conjunction with the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic representation of a three axis measuring machine.

FIG. 2 is a diagram illustrating the shifting of the switching point ofthe switching sensor as a function of the sensing speed.

FIG. 3 is a block diagram of a presently preferred embodiment of theapparatus of this invention.

DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EMBODIMENTS

Turning now to the drawings, FIG. 1 represents a three axis measuringmachine having a table T which serves to support a test object P whichis shiftable in three orthogonal machine axes with respect to a machinebase B. A length measuring arrangement L is provided on the three axismeasuring machine for measuring the displacement of the table T withrespect to the test object P in the X direction. For simplicity, onlythe X direction will be considered in the following discussion, althoughit should be understood that a similar approach can be used for the Yand Z directions as well. A switching sensor S acting in the threemachine axes is secured to the machine base B for the sensing ofsurfaces F₁, F₂ of the test object P.

In order to determine the position and/or the outside measurements ofthe test object P in the X direction, the table T with the test object Parranged on it is shifted in the X direction with a certain prescribedspeed until the surface F₁, F₂ to be measured of the test object P comesinto contact with the sensing ball K of the switching sensor S. Theposition of the table T and thereby of the test object P is measured bythe length measuring arrangement L. At the moment of contact between thesurface F₁ to be sensed of the test object P and the sensing ball K ofthe switching sensor S, the switching sensor S gives off a signal whichcauses the measuring value present at this moment in the lengthmeasuring arrangement L for the relative position between the testobject P and the switching sensor S to be stored. Thereafter in the samemanner the surface F₂ lying opposite the surface F₁ of the test object Pis sensed by the switching sensor S. From the two measuring valuesthereby obtained the position and outside dimension of the test object Pin the X direction are determined.

The table T is movable by means of a spindle SP which is rotated by adrive unit AT in order to alter the relative position between the testobject P and the switching sensor S. The drive unit AT is controlled bya numerical control arrangement ST via a digital to analog converter Dand a drive regulator AR, as shown in FIG. 3. The length measuringarrangement L for measuring the displacement of the test object P withrespect to the switching sensor S is coupled to an up-down counter Zwhich counts with correct sign (plus or minus) measuring impulsesgenerated by the length measuring arrangement L. The switching sensor Sgenerates a switching signal at the moment of contact between thesurfaces F₁, F₂ to be sensed of the test object P and the sensing ballK. This switching signal is applied to the control arrangement ST, andthis switching signal causes the up-down counter Z to retain theinstantaneously prevailing measuring value and to transfer thismeasuring value to an adder AD. The control ST responds to the switchingsignal by halting relative movement between the switching sensor S andthe test object P.

FIG. 2 represents a diagram of the displacement S of the switching pointof the switching sensor S as a function of the sensing speed V for apredetermined material pairing as a result of deformations between thesensing ball K of the switching sensor S and the test object P at thecommon point of contact.

According to this invention, switching point displacements S of the typedescribed above in conjunction with FIG. 2 are stored as previouslydetermined correction values as a function of the scanning speed V inthe form of a correction table in digital form in a memory M₁ which isconnected with the adder AD and the control arrangement ST. Similarly,previously determined correction values for the various materialpairings are stored in digital form in a memory M₂ which is likewiseconnected with the adder AD as well as to an input unit E. This inputunit E operates to select the appropriate correction value for theparticular material pairing present in a particular measuring operationfrom the correction values stored in the memory M₂. This selected one ofthe correction values of memory M₂ is applied as an input to the adderAD. The control arrangement ST selects on the basis of the sensing speedV prescribed by it the correction value from the memory M₁ appropriateto the particular sensing operation. This selected correction value fromthe memory M₁ is also applied as an input to the adder AD.

The adder AD also receives as an input the measuring value transferredfrom the counter Z at the moment the switching signal generated by theswitching sensor S at the moment of contact is received. The adder ADoperates to add this measuring value with the selected correction valuesfrom the memories M₁, M₂ in order to correct for sensing errorsresulting from sensing speed and the particular material pairing betweenthe ball K and the test object P. The resulting corrected measuringvalue is then stored in a memory M₃ and can thereupon be displayed bythe control arrangement ST in a display unit A. From the two correctedmeasuring values in the scanning of the surfaces F₁, F₂, there isobtained the position and outside dimension of the test object P in theX direction.

The adder AD can be formed (in a manner not shown) by a computerincluded in the control arrangement ST, by which there can also occur afurther processing of the corrected measuring values stored in thememory M₃.

The correction values can be determined in advance as a function of theparticular discreet variable sensing speeds to be used, or alternatelythe correction values can be allocated in each case to a predeterminedrange of sensing speeds.

In this specification the term "material pairing" is meant to refer tothe material properties of the sensing ball K of the switching sensor Sand of the test object P to be sensed in respect to their hardness,i.e., their resistance to mechanical deformation at the common point ofcontact in the sensing operation.

The velocity dependent error corrected for by means of the memory M₁arises as a function of the velocity of approach between this switchingsensor S. This is because after the switching sensor has contacted thetest object P, the table T continues to travel a short distance beforethe presence of the test object P is sensed due to the deformation ofthe sensor ball K and the test object P. The magnitude of this distanceis dependent upon the velocity of approach and is stored in the memoryM₁ for correction purposes.

The correction values stored in the memory M₂ to correct for errorsresulting from material pairing of the test object P and the sensingball K arise due to deformation of at least one of the test object P andthe sensing ball K during the sensing process. These correction valuesare dependent upon the particular material pairing used for the sensingprocess and are stored in the memory M₂.

The velocity dependent errors and the material pairing errors canaccumulate. That is, velocity dependent errors and material pairingerrors can occur simultaneously and for this reason in the preferredembodiment described above the correction values stored in the memoriesM₁, M₂ are superimposed on one another. In alternate embodimentscorrection values from the memories M₁, M₂ can be multiplied togetherrather than added together in order to perform the desired correction.

Of course, it should be understood that a wide range of changes andmodifications can be made to the preferred embodiments described above.It is therefore intended that the foregoing detailed description beregarded as illustrative rather than limiting, and that it be understoodthat it is the following claims, including all equivalents, which areintended to define the scope of this invention.

I claim:
 1. In a process for measuring the position and/or dimensions ofa test object with at least a measuring system coupled to a switchingsensor, wherein the switching sensor is movable relative to the testobject with an approach velocity, wherein the measuring system generatesa measuring signal indicative of the relative position of the switchingsensor with respect to the test object, and wherein the switching sensorgenerates a sensing signal upon contact with the test object, theimprovement comprising:storing a set of correction values indicative ofpreviously determined switching sensor errors, said errors resultingfrom the velocity of approach between the test object and the switchingsensor, and the material characteristics of the switching sensor and thetest object; utilizing the sensing signal to store a value of themeasuring signal; and correcting the value of the measuring signal withat least a selected one of the correction values to substantiallyeliminate errors said at least a selected one of the correction valuesselected in accordance with said at least one of (1) the velocity ofapproach between the test object and the switching sensor and (2)material characteristics of the switching sensor and the test object. 2.The method of claim 1 wherein the correction values are stored in atleast one memory.
 3. The method of claim 1 wherein the correction valuesare determined in advance in accordance with a selected velocity ofapproach between the test object and the switching sensor.
 4. The methodof claim 1 wherein the correction values are divided into sets, eachallocated to a respective range of velocities of approach between thetest object and the switching sensor.
 5. The method of claim 1 whereinthe correcting step comprises the step of adding the selected one of thecorrection values to the value of the measuring signal.
 6. The method ofclaim 5 wherein the adding step is performed by a computer included in acontrol system which controls movement of one of the switching sensorand the test object.
 7. The invention of claim 6 wherein the correctingvalues comprise first correcting values indicative of switching sensorerrors resulting from the velocity of approach between the test objectand the switching sensor, and stored in a first memory; wherein thecorrecting values comprise second correcting values indicative ofswitching sensor errors resulting from material characteristics of theswitching sensor and the test object, and stored in a second memory;wherein the control system selects one of the first correcting valuesfrom the first memory for use in the correcting step, and wherein aninput unit selects one of the second correcting values from the secondmemory for use in the correcting step.
 8. The method of claim 7 whereinthe control system selects said one of said first correcting values as afunction of the velocity of approach between the test object and theswitching sensor.
 9. The method of claim 1 further comprising the stepof storing the corrected value of the measuring signal.
 10. In ameasuring apparatus of the type comprising a switching sensor movablerelative to a test object with an approach velocity and operative togenerate a switching signal upon contact with the test object; and atleast a measuring instrument operative to generate a measuring signalindicative of the relative position of the switching sensor with respectto the test object; the improvement comprising:a first memory forstoring first correction values indicative of switching sensor errorsresulting from the velocity of approach between the test object and theswitching sensor; a second memory for storing second correction valuesindicative of switching sensor errors resulting from materialcharacteristics of the test object and the switching sensor; and anadder coupled to the measuring instrument and the first and secondmemories for adding a selected one of the first correction values and aselected one of the second correction values to the measuring signal inorder to correct for switching sensor errors.
 11. The invention of claim10 further comprising:control means for controlling the position of theswitching sensor relative to the test object, said control means coupledto the first memory to select the selected one of the first correctionvalues; and an input device coupled to the second memory to select theselected one of the second correction values.
 12. In a measuringapparatus of the type comprising a switching sensor movable relative toa test object with an approach velocity and operative to generate aswitching signal upon contact with the test object; the switching sensorand test object being subject to deformation upon contact with the otherand each capable of being comprised of at least one of a plurality ofpreselected materials; and at least a measuring instrument operative togenerate a measuring signal indicative of the relative position of theswitching sensor with respect to the test object; the improvementcomprising:a first memory comprising a table of predetermined firstcorrection values representative of the deformation of the switchingsensor and of the deformation of the test object as a function of theapproach velocity for a preselected combination of switching sensormaterial and test object material; a second memory comprising aplurality of second correction values representative of thecharacteristics of the combination of the selected material of theswitching sensor and the selected material of the test object; and acorrection circuit responsive to the switching signal and coupled to themeasuring instrument and the first and second memories for applying acorrection to the measuring signal based on a selected at least one ofthe first correction values and a selected at least one of the secondcorrection values to correct for errors in the measuring signal causedby the deformation of the switching sensor and the test object.